1. Field of the Invention
The present invention relates to a method for separating color electronically and an apparatus therefor which are applied to record color separated images of Yellow (Y), Magenta (M), Cyan (C) and Black (B) in the platemaking process for color printing operation by using an image reproduction scanner.
2. Prior Arts
Conventionally, color expressions in the color printings have been carried out by using, in general, three kinds of inks such as Y, M and C color inks. However, with only these color inks, resultant color prints are liable to lack in density range, so that K ink has been auxiliarily used conventionally.
Contrary to such as a black printing type called as "Skelton-black", there exists another type of printing neutral color component in which K ink is used as possible by substituting neutral color component called to be "Full-black" for K ink.
There can be numbers of printing types of black print by adjusting ratio of amount of neutral color component which is replaced with K ink. In the cases of the "Full-black" printing type and these intermediate printing types, according to an amount of K ink to be printed, amounts of the three color inks, i.e., those of Y, M and C inks to be printed must be reduced comparing with that of "Skelton-black". This is called to be "Under Color Removal" (UCR).
The more approaches to a printing type of the black print of the "Full-black", the more the total amount of inks to be printed is reduced, and that expensive Y, M and/or C color inks can be replaced with less expensive K ink, so that cost for the printing inks can be reduce, and further, it is possible to reproduce neutral color easily which results in facilitating the printing operation. However, on the other hand because of the fact that there are some difficulties such as estimation of resultant color separation and other reasons, the black print of the "Full-black" or those of intermediate ones near the "Full-black" have scarcely been used in practical, accordingly, commonly the "Skelton-black" has been applied.
However, recently, the above described advantages of the "Full-black" have been recognized, and a method applying larger UCR, that is, a printing method in which neutral color components in Y, M and C color inks are replaced with K ink with a higher ratio has been pervaded. This method is, briefly, to be able to reproduce any minute unit areas in a color printings (color(s) to be reproduced) with one color (i.e., K) or three colors (i.e., two colors among the three colors, Y, M and C and K), if neutral color component of color(s) to be reproduced in the areas is completely replaced with K ink. Accordingly, it means that according to the present method, in principle, all colors in the color printings can be reproduced at the maximum by three kinds of color inks among four colors, i.e., Y, M, C and K. If neutral color component included in three color inks Y, M and C is replaced with a kind of K ink, so that it is possible to reduce the amount of expensive color inks which results in achieving wide cost down and easy controlling of printing operation.
However, in color separation works by a usual image reproduction scanner, at an area in which the amount of three kinds of color inks of Y, M and C, that is, at an area where neutral color density is rather higher, if all the amount of neutral color component of the three color inks is simply replaced with K ink, in general no good quality of printings can not be obtained. This is because of color density to be reproduced in the case of merely K ink being printed independently is lower than that of in the case in which four color inks consisting of Y, M and C three color inks and K ink are overlapped.
In order to improve the afore-mentioned disadvantage, a prior patent application (Japanese Patent Application Nos. 58-249542 and 58-249543; Title of the Invention: "METHOD FOR COLOR CORRECTION"; inventors Yamada et al, filed in the U.S.A. on Dec. 21,1984) by the very Applicant in which a method for compensating for deficient density of K ink is provided. In addition, in the Japanese Patent laid-open Publication Nos. 57-173838 (German Patent Application No. (P31 09 190.3) and 58-190951 (German Patent Application No. P32 10 309.5) there are disclosed means which increase amount of K ink to be printed without failing gray balance, and contrary thereto means for reducing three color inks, Y, M and C to be printed are also disclosed.
However, actually it is very complicate to replace an amount equivalent to neutral color density with that of K ink, and carry out subtraction the amount from that of the three color inks, if impurity characteristics in the three color inks, Y, M and C, density characteristics of K ink etc. are considered. That is, each of the color inks includes some of other color components, i.e., some impure components, so that change of the amount even only one color ink influences consequently upon the amount of other two inks.
As the result thereof, correction by means of adjustment of the amount of inks as described the above is to be repeated and a color correcting operation circuit of extremely complicated construction and complex handling are required for carrying out the works. Further, in the color correcting operation above mentioned parameters of components of impurity in each of the above-described color inks are operated as a value which proportionates to each of color density values. In actual, however, there exists "Breach in Proportional Law" which regulates that the amount of impurities does not directly proporionate to that of color inks. In addition, there also exists another law "Breach in Summing Law" which means that in the case of plural kinds of color inks being printed overlappedly, resultant density is produced or represented lower than the sum of each of color ink densities. Accordingly, considering even these phenomena, construction of the color correction operating circuit and handling thereof become more complicate to perform color correcion. Thus, practicability of the above-mentioned is quite low.